The acetylcholine receptor (AChR) in skeletal muscle is normally confined to the endplate membrane, but during development and after denervation, AChRs also occur over the entire surface of muscle fibers. These extrajunctional receptors (EJR) differ in their functional biochemical and immunological properties from junctional receptors (JR), and are thought to play a role in the early stages of synapse formation. Our research is aimed at understanding how the two receptors are related biochemically, how their functional properties are determined, and what roles the two receptors play during synaptogenesis. Another aim of our work is to understand how antibodies affect the turnover of the two receptor types of experimental autoimmune myasthenia gravis (EAMG). We have previously shown that patients with myasthenia gravis contain antibodies that react only with rat muscle EJR. We will use monoclonal antibodies and the specific antibodies in myasthenia sera to investigate the properties of the AChR during synapse development. We will try to relate the molecular changes that occur with functional changes in the receptor. Because EJR may be precursors for JR during development, we will use specific antibodies to seek enzymatic activities that interconvert the two receptors. We will also investigate the role of the nerve in the developmental changeover from EJR to JR. To investigate the role of membrane environment in determining receptor properties, we will transfer Torpedo AChR into muscle membranes and compare the properties of endogenous and exogenous receptors. Antibodies to the AChR increase the metabolic turnover of both JR and EJR, an effect which could explain the reduced number of AChRs in myasthenic patients. We will investigate the antigenic specificity required for this modulation by making and characterizing monoclonal antibodies to rat muscle AChR. We wish to discover what factors determine the ability of an antibody to affect receptor turnover and to cause EAMG in animals. Finally, we will investigate AChR from human muscle to determine if it has junctional and extrajunctional forms and will use the human muscle AChR to characterize myasthenic sera.